1. Field of the Invention
The present invention relates to a technology to detect a load power supply current in a power supply apparatus that supplies direct current power to a load with a power supply unit.
2. Description of the Related Art
As shown in FIG. 4, a conventional power supply apparatus 500 includes a transformer 501, a rectification-smoothing circuit 502, a switching element 503, a control integrated circuit (IC) 504, a detection circuit 505, a load current detecting circuit 506, and an overcurrent setting switching circuit 507. See, for example, Japanese Unexamined Patent Application Publication No. 2008-172973, especially paragraphs [0031] to [0041], FIG. 1, etc. In the power supply apparatus 500, the voltage of direct current power supplied from an input terminal to a primary side of the transformer 501 is converted into a certain voltage value by the transformer 501, and a secondary voltage of the transformer 501 is rectified and smoothed by the rectification-smoothing circuit 502 and is supplied from an output terminal to an external load.
A primary current and a primary voltage of the transformer 501 are controlled by switching turning on-off of the switching element 503 by the control IC 504 on the basis of the current (voltage) detected in the detection circuit 505. The power supply apparatus 500 is configured so that the circuit configuration of the detection circuit 505 is switched by the overcurrent setting switching circuit 507 on the basis of a load current detected by the load current detecting circuit 506 to vary a detection voltage to be input into an over current latch (OC) terminal of the control IC 504 in order to switch the setting value for detection of the overcurrent.
Accordingly, since the setting value in the detection of the overcurrent by the detection circuit 505 can be switched by the overcurrent setting switching circuit 507 on the basis of the value of a normal load current detected by the load current detecting circuit 506 including a detection resistor 508, it is possible to detect any overcurrent that occurs even when the load current is much smaller than a rated output current, thereby surely preventing the elements in the power supply apparatus from, for example, being burned out. FIG. 4 is a diagram showing an exemplary power supply apparatus according to the related art.
Electronics including notebook personal computers, mobile phones, mobile audio players, and flat-panel liquid crystal display televisions are increasingly reduced in size in recent years and the power supply apparatuses installed in such electronics are required to be further reduced in size. However, the load current is detected on the basis of a terminal voltage of the detection resistor 508 in the load current detecting circuit 506 and the overcurrent is detected on the basis of a terminal voltage of a detection resistor 509 in the detection circuit 505 in the above technology according to the related art, it is necessary to provide the detection resistors 508 and 509 for current detection and to ensure areas where the detection resistors 508 and 509 can be mounted.
In addition, a technology to detect the load current by using a series parasitic resistance of a smoothing inductor provided on a power feed line as the detection resistor is also known. However, in this case, although it is not necessary to provide the detection resistor, it is necessary to provide components including resistors and capacitors for compensating for the electromotive force based on the inductance, and to ensure areas where such components can be mounted.